Development and Characterization of Novel Refractory High-Entropy Alloys for High Temperature Applications

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Nowadays, Ni-based superalloys are the dominant material class for high temperature applications. However, their use is strictly limited by their melting point of around 1400°C. In this project, so called High Entropy Alloys (HEA), which are based on refractory metals, will be proposed as novel high temperature structural materials. The appealing difference of the concept of HEA is that all alloy elements exhibit equimolar or near-equimolar concentrations. Thus, from a simplified thermodynamic point of view, the formation of random solid solutions being potentially ductile phases is favored against intermetallic compounds, which are often complex and brittle in nature. As a further consequence, this simple microstructure may yield an attractive property profile such as high thermal stability and high temperature strength while being tough and even ductile at ambient temperatures.First results clearly show that the alloy 20Mo-20W-20Al-20Cr-20Ti possesses a single phase microstructure, which is body centered cubic. Although many materials containing refractory metals suffer from severe oxidation, the alloy studied reveals a surprisingly good high temperature oxidation resistance. Besides, the calculated melting point is approximately 300°C higher than that of commercial Ni-based superalloys. In this proposal, new HEA based on the system Mo-W-Al-X-Y will be developed. Three reference alloys with the following chemical compositions are chosen for further investigations: (i) 20Mo-20W-20Al-20Cr-20Ti, (ii) 20Mo-20W-20Al-20Cr-20Mn und (iii) 20Mo-20W-20Al-20Mn-20Ti. Since the HEA approach allows micro-alloying, the effect of several additional elements, such as B, Zr, Si, and Y will be studied. Well known from past experience with conventional alloys, the addition of B and Zr may lead to a significant improvement of mechanical properties, whereas alloying with Si and Y may cause a substantial increase of the oxidation protectiveness. This proposal mainly aims at the exploration of potential of novel refractory HEA and the development of physical metallurgy strategies to satisfy the requirements for high temperature applications. Experimental investigations will be focused on extensive microstructural analysis, creep behavior, and high temperature oxidation resistance. Based on this, metallurgical approaches on targeted alloy development can be derived which may facilitate property improvement with respect to future high temperature applications.